CTSS and pulmonary emphysema: Proteolytic degradation or modification of ECM proteins (e.g., proteoglycans, fibrillar collagens, glycoproteins, elastin) may generate new antigens that bind and activate immune cells in lungs,27–29 which can ultimately lead to emphysema.30,31 In our results, 26 active proteases and 38 protease-inhibitor proteins changed significantly after SARS-CoV-2 infection, including the ECM regulators, members or the serpin family, cathepsins, and matrix metalloproteinases (MMPs) (Fig. 4e).